The static weight distribution front vs rear locates the centre of gravity in plain view. The position of the centre of gravity is very important in determining the balance of the car, as we shall see in this article.
For a new race car, an important early decision in the design process is the choice of front to rear weight distribution.
Equally, if we are racing an existing car, it’s important to know what our weight distribution is, so we can make decisions about the suspension set-up of the car.
How do we measure weight distribution?
If we have the corner weights, we can determine the weight at the front and rear axles as a proportion of the total weight. E.g. We may have 53% of the total weight at the front axle, leaving 47% of the total weight at the rear axle.
How does weight distribution affect the balance of the car? What is the effect of more front weight? Or more rear weight?
To get answers to these questions, let’s look at the problem from the perspective of a diagram we have developed at Racing Car Technology, our schematic diagram of handling. Looking down from above, it shows the the forces and so-called "moments" acting on the vehicle in the ground plane.
Rather than showing individual lateral force at each tyre contact patch, we can more conveniently show the lateral force for the wheel pair i.e., one lateral force per axle, FF at the front and FR at the rear.
The front yaw moment is (front axle force, FF x a), and the balancing rear moment, acting in the opposite direction is (rear axle force, FR x b).
When the racing driver turns the steering wheel, the steered angle of the front tyres and the forward momentum of the car build a side force, FF, at the front axle. This results in a net yaw moment at the front, turning the car into the corner. The rotation at the front of the car is followed quickly by a balancing yaw moment at the rear.
While the driver applies the steering input, there is a net yaw moment being applied around the centre of gravity to turn the car into the corner. Considering the forward momentum of the race car at speed is a very large number, we can visualise the torque required to get the race car to turn as hard as possible is also quite a large number.
The rear yaw moment, (FR x b), is removing or offsetting the yaw moment at the front of the car. So, as the car settles in the corner, the effect of the driver’s initial input in turning the car is reduced to zero. At that moment, the car is in “steady state” cornering, i.e., fixed steering and throttle, awaiting the next input from the driver.
There is no yaw moment being applied to maintain steady state cornering. The car can continue driving in a circle at a fixed speed and fixed yaw rate.
For the driver to apply just the right amount of torque needed in the corner entry, we must have feedback from the car that can gives the driver a feeling of whether the more or less torque is needed. This feedback to the driver is via the driver's sensitivity to rotation, the driver's ability to feel a change in rotation in the oversteer direction.
If the net effect of the set up and driver technique adds to the front turning moment, we are building more agility (oversteer).
If the net effect of the set-up and driver technique reduces the front turning moment, we have more stability (understeer).
The influence of increasing agility is that the car will turn better. If the car gets to be too agile (too much oversteer), the car will be “nervous” and almost every motion of the car in cornering requires a driver correction.
The influence of increasing stability is that car will be, to at least some extent, more self-correcting. Stability adds to driver confidence. When the car is too stable (too much understeer) the car will not turn well.
Weight Distribution and the Balance of the Race Car
Looking at our schematic diagram, if front and rear moments are balanced, i.e., FFxa = FRxb, we say the car is in “steady state” cornering. The car is travelling on a fixed radius with fixed throttle and steering.
If the centre of gravity is positioned at the midpoint between front and rear axles, then a = b. Given a = b and if we have the same tyres all round, then front and rear tyres will reach their maximum grip at the same time. At the limit therefore, the car can generate the maximum mid-corner lateral G available from the tyres. Front and rear tyres reach the limit of grip at the same time.
If the centre of gravity is forward of the midpoint between the front and rear axles (like many front engine production cars), then a is less than b.
The front moment has a shorter lever arm, and the rear moment has a proportionally longer lever arm.
This means for the front and rear moments to be balanced, in steady state cornering where FFxa= FRxb, lever arm a is shorter, and lever arm b is longer:
Front lateral force, FF, must be greater than rear lateral force, FR. i.e.
For a car with higher front weight percentage, the front tyres will reach the limit of grip first, leaving some unused grip availability at the rear of the car. Before doing anything to adjust the set-up, the car will be a natural understeerer.
If the centre of gravity is rearward of the midpoint between front and rear axles e.g., mid-engine race cars, then b is less than a. The rear moment has a proportionally longer lever arm than the front.
So, for the car to be balanced we have the opposite of the front heavy car. FR>FF.
For a car with higher rear weight percentage, the rear tyres will reach the limit of grip first, leaving some unused grip availability at the front. The car will be a natural oversteerer.
What Happens When We Add Weight to the Race Car?
It should be noted we never want to add weight to our race car. Given that the chassis and suspension has the required stiffness to maintain best grip, the lighter race car should always be faster. Better acceleration, braking and cornering. So, for this exercise just consider we are adding weight to get to a minimum weight required for our category of racing.
We could add the weight either forward or rearward of the existing centre of gravity.
If we are adding weight, we should be aware we are changing the location of the centre of gravity front to rear.
Our first inclination is to think we are going to help stick the end with the additional weight.
For acceleration and braking this is true. The rear engine car with rear weight bias is always going to launch off the line better than a front engine car. thinking for braking.
Also, the rear engine car has front and rear tyres more evenly loaded under brakes compared to the front engine car. You can adjust the brake bias more towards the front of the car to take advantage of the extra grip available at the front, and also to reduce the possibility of locking up the rears.
Hands up if you've ever carried this thinking on to cornering capability as well. I think everybody will have their hand up, including me.
We are inclined to think, because adding vertical load, or weight, increases grip at the tyres on the axle in question, that this will increase the cornering capability at that end of the car. We might think about "sticking the end with the extra weight." This works for acceleration and braking, but not for cornering.
For cornering (lateral acceleration), it depends on what happens to the yaw moments. From our understanding of the vehicle dynamics model of handling, as shown in our schematic diagram, the effect of adding weight to the front or rear of the car is the exact opposite of "sticking the end with the extra weight":
Adding weight forward of the centre of gravity will increase understeer. Adding weight rearward of the centre of gravity will increase oversteer.
For 2020/21, at Racing Car Technology, we've decided to put the vehicle dynamics simple model (our schematic diagram above) front and centre in our thinking when looking at vehicle dynamics problems. The proof and demonstration of the value of the vehicle dynamics model goes back decades. If an idea seems right according to the model, there’s a big chance it is right ahead of any thinking to the contrary.
The mathematical model itself is what drives engineering design work and race car simulation. If we see it working for the engineers, it should work for us also, giving us confidence in our conclusions.
But of course, without the ability to do the maths, we do have to interpret carefully. I’m thinking some of those with more in-depth knowledge of the vehicle dynamics model might have better explanations.
50-50 Weight Distribution
For a high-performance road car, what is the ideal weight distribution?
For RWD front engine, 50-50 weight distribution. It’s the layout of choice for RWD performance cars from BMW, Mercedes etc for many years. By 50-50 we mean the centre of gravity (where the weight of the vehicle is concentrated) is at a point midway between front and rear axles.
50-50 cars display a degree of natural agility yet with a certain calmness that aids driver confidence. It’s pretty easy to get the final balance you want with correctly sized anti-roll bars front and rear.
Front Engine, Rear Wheel Drive
With front engine RWD production cars for racing, at Racing Car Technology we find we can still get good balance with weight distribution between 53% front weight percentage and 47% front weight percentage. (We set the baseline balance of the car with the Racing Car Technology Weight Transfer WorkSheet™.) If we get to 55% and greater front weight percentage, we’re giving away some handling performance.
In a recent Car and Driver comparo between the Mustang GT500 and Camaro ZL1, it was stated the Mustang has 55% + front weight percentage and the Camaro closer to 50-50. According to the article, the Camaro is the better handling car. It seems the high front weight percentage of the Mustang is a result of different design compromises between the two cars. A high front weight percentage would be something for the Mustang handling package to overcome as it is for some other sports cars such as the Toyota 86.
What’s the point of having a larger rear weight percentage? For circuit racing, it aids agility and with more weight over the rear axle, gives better straight-line acceleration.
Mid-Engine and Rear Engine
The preferred layout for circuit racing and super high-performance cars on the road is mid-engine.
Racing cars are likely to have a high power to weight ratio and could therefore be grip limited at the rear. More rear weight percentage is desirable to add vertical load to the rear tyres and therefore create more grip for straight line acceleration.
As we have seen more rear weight adds to agility. The mid-engine car will turn better. (The idea that the car could turn better with more weight over the front wheels doesn’t stack up, as discussed earlier).
It almost goes without saying - any cornering performance improvement will trump just about any associated downside. This is true for aero downforce, for instance. The gain in cornering performance from the downforce is worth way more than the loss in straight line performance from the aero drag.
When first introduced in the 50’s and 60’s, mid-engine race cars dominated F1 and the Indianapolis 500 straight off, with no appreciable downsides to consider. The mid-engine layout was a no brainer.
If we have the same tyres all round, the limit in rear weight percentage is around 55%. This would be the case for most Formula Fords, for instance. But with wider section rear tyres and high power to weight, rear weight is most likely best around 60%.
Especially with rear engine cars such as the Porsche 911, we have difficulty getting away from the idea that the rear weight is going to help lateral grip at the rear. Motoring journalists perpetuate the myth with comments like this: In a recent comparo between the earliest and latest 911 GT3s, the author says of the latest GT3, “Turn-in, typically a 911 weakness, is incredible thanks to the wider front track, rear wheel steering and aggressive geometry”.
When we consider the high rear weight percentage on a 911, we might conclude the exact opposite. Early model 911s turned in too aggressively. Perhaps the rear wheel steering and geometry tweaks of the late model car make more cornering power by calming down the aggressive behaviour of the 911 layout at critical points in the corner.
Front Wheel Drive
Since the Mini, front wheel drive has been popular in racing. Even more so today with all the hot hatches on the market. With the advent of TCR touring cars, we have a mainstream front wheel drive only category for the first time.
Around 60% front weight would appear to be the norm. In fact, TCR have mandated 60-40 weight distribution in the rules.
If you were allowed a larger tyre size on the front, it would be a distinct advantage. But with the same tyre all round some slightly lesser front weight than 60-40 may be an advantage. In 2 litre touring cars some years ago, the teams would modify the weight distribution by rolling the engine back in the chassis. The cost was prohibitive, and the gain was small – around 2 to 3 % gain in rear weight percentage.
There’s more to the simple vehicle dynamics model than what we can cover here. See our E Book for more details and an overview of all the training we offer on how to use the ideas to tune your suspension - “7 Little-Known Hacks to Suspension Set Up Mastery.”